System and method for ground based inspection of wind turbine blades

ABSTRACT

A ground based wind turbine blade inspection system and method consists of a thermal imaging camera configured to detect propagating defects by acquiring thermal imaging data from a wind turbine blade when it is substantially at thermal equilibrium with respect to surrounding air and analyzing the thermal imaging data with a processor to identify thermal effects associated with latent defects caused by internal friction due to cyclic gravitational stresses and wind loads during normal turbine operation. The system permits latent defects to be identified using a ground-based in situ inspection before they become visually apparent, which allows repairs to be made economically while the blade is in place.

INCORPORATIONS BY REFERENCE

Applicant hereby incorporates by reference, as if set forth fullyherein, the entirety of the disclosures of U.S. Nonprovisional patentapplication Ser. No. 13/731,085, filed Dec. 30, 2012 and having thetitle METHOD AND APPARATUS FOR THE REMOTE NONDESTRUCTIVE EVALUATION OFAN OBJECT, U.S. Nonprovisional patent application Ser. No. 13/837,145,filed on Mar. 15, 2013 and having the title METHOD AND APPARATUS FORMONITORING WIND TURBINE BLADES DURING OPERATION and U.S. Nonprovisionalpatent application Ser. No. 13/840,470, filed on Mar. 15, 2013 andhaving the title NONDESTRUCTIVE ACOUSTIC DOPPLER TESTING OF WINDTURBINES BLADES FROM THE GROUND DURING OPERATION.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention describes method and apparatus for inspecting wind turbineblades on rotating wind turbine generators, from the ground. Theinvention has utility for remote detection of propagating latentdefects, existing damage and broken adhesive bonds within the skin of awind turbine blade. This permits subsurface defects to be detectedbefore they become too large for repair in situ, which providesignificant economic advantages, as the cost of repairing the windturbine blade in situ is typically 10% of the cost of replacing a blade.

2. Description of the Related Technology

Due to their large size, extensive surface area and complex shape, windturbine blades are difficult to non-destructively inspect even within afabrication or repair facility. Visual inspection cannot identifydefects below the surface of the outer skin of the wind turbine blade,which typically is fabricated from a fiberglass material. Activethermography inspection techniques are effective for near surfacedefects but can give false positives and false negatives due tovariations in material thickness and surface emissivity. Angle beamultrasonic techniques are very slow and may not work through thickcarbon fiber spar caps. As a result, blades are commonly installed ontowers and put into service with a significant probability of latentmanufacturing defects. Furthermore, composite blades are subject toextreme temperature variations. Entrapped water in blades can undergofreeze/thaw cycles, which can cause internal damage. Cyclic forces ofgravity and varying forces from the wind acting on the blades as theyrotate can cause fatigue damage or the propagation of latent defectsover time while manufacturing process mistakes can lead to early bladefailure. Defects can grow below the surface of a wind turbine blade tothe point that by the time cracks and damage breach the surface and canbe detected visually, the damage may not be repairable on tower.

Detecting progressive subsurface damage and propagating defects in windturbine blades in situ is difficult for a number of different reasons.Inspectors using sky cranes or rope access are expensive, time consumingand put personnel in a very dangerous working environment. While ontower, close access allows inspectors to visually detect blade defectssuch as trailing edge splits, cracks, lightning damage and bladeerosion. In addition, major subsurface delaminations, cracks, debondedof adhesive joints can easily go undetected with current technology.

Access to a wind turbine blade in situ with portable instruments fornondestructive testing also requires rope access or sky platforms andcranes. Blade and tower crawlers with nondestructive testing sensors forin situ inspection have been developed and tested, but they can beprohibitively expensive, slow to operate, require repair and maintenancethemselves. Their effectiveness is also questionable. Microwave andradar scanners, while effective for dielectric materials do not work oncritical components such as spar caps manufactured that haveelectrically conductive carbon fiber materials.

There accordingly exists a need for a fast, cost effectivenondestructive inspection system and method for wind turbine blades todetect latent and propagating damage early enough to allow on-towerrepair before it becomes necessary to remove the wind turbine blade fromthe tower and repair it off-site or replace it with a new blade.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a fast, costeffective nondestructive inspection system and method for wind turbineblades to detect latent and propagating damage early enough to allowon-tower repair before it becomes necessary to remove the wind turbineblade from the tower and repair it off-site or replace it with a newblade.

In accordance with the embodiment described herein, a system forinspecting utility scale wind turbine generator blades from the groundfor propagating subsurface anomalies during normal operation, comprisedof a camera sensitive to the thermal emissions from defects subjected tocyclic strain from gravity and wind loading and a means to processthermal images of these emissions to determine location, signal to noiseratio, size or other quantitative measurements and that such earlydetection may allow repair of the blade up tower instead of more costlyreplacement.

A second embodiment includes a means to stabilized the thermal images ofa wind turbine blade over at least several successive video frames toreduces the image degrading effects of blade motion.

A third embodiment includes a means of stabilizing the camera whenmaking inspections of off shore wind turbines from a boat.

These and various other advantages and features of novelty thatcharacterize the invention are pointed out with particularity in theclaims annexed hereto and forming a part hereof. However, for a betterunderstanding of the invention, its advantages, and the objects obtainedby its use, reference should be made to the drawings which form afurther part hereof, and to the accompanying descriptive matter, inwhich there is illustrated and described a preferred embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a utility scale, horizontalaxis, wind turbine generator.

FIG. 2 is a schematic representation of a wind turbine blade crosssection.

FIG. 3 is a schematic showing the relative location of the thermalcamera for inspection the wind turbine blades then orient approximatelyparallel with the ground or sea level.

FIG. 4 shows a plan view of a wind turbine site with six locations forthe thermal camera to view various locations on the blades.

FIG. 5 shows a basic thermal imaging set up suitable for direct viewingand recording blade surfaces in or near the turbine blade disk.

FIG. 6 a schematic showing the thermal camera viewing a low pressure, orsuction side of a blade in the horizontal position.

FIG. 7 shows how an electronic image peak store unit captures thetrajectory of thermal emitting defects as the blade passes through thethermal camera field of view.

FIG. 8 is a schematic diagram of a manually controlled moving mirrorassembly mounted in front of the thermal camera to stabilize, orderotate, thermal images of a rotating turbine blade and suitable foruse when the blade is in approximately a horizontal orientation.

FIG. 9 is a schematic of computer controlled moving camera imagestabilizer for derotating a sequence of thermal images of a rotatingturbine blade using the presence of the blade in the video image totrigger the start of blade tracking.

FIG. 10 shows the timing of the ramp voltage waveform used to drive thelinear motor to drive the hinged plate supporting the thermal camera totrack one blade only turing with a period of four seconds.

FIG. 11 is a schematic diagram of a gimbal mounted thermal camera fortesting off shore wind turbine blades from a boat or ship.

FIG. 12 is a schematic diagram of a thermal camera testing a blade witha visible light camera and light source to image the serial number ofthe blade and showing the image of the serial number presented with thetest images of the blade.

FIG. 13 is a photographic depiction of image frames showing testingperformed according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The embodiments of the present invention disclosed herein, describes asystem and method for the nondestructive inspection of wind turbineblades suitable for both onshore and off-shore wind power generators andcapable of detecting propagating defects or damage during normal turbineoperation. Inspections are made from ground or sea level without anyrequirements for access to the wind generator tower or any disruption ofpower generation. Large utility scale wind power turbines are generallyof the HAWT design using composite air foil shaped blades to generatethe rotational torque needed to drive the electrical generator. Currentutility scale wind turbine blades may range from 9 m in length up tomore than 50 m, with much larger blades being designed for offshore windpower generators. The application of this invention may achieve goodresults on blades of all lengths manufactured with thermoelasticcomposite materials such as fiber glass and carbon fibers in an epoxymatrix.

Wind turbines blades, during normal operation, are subjected tocontinuous cyclic loading due to gravity and variable wind forces.Thermal imaging of blade anomalies requires the test be conducted aftersunset or on cloudy days during normal operation. After the blades havecome to thermal equilibrium with the ambient air temperature the onlyremaining thermal emissions from the blade occur at the site ofpropagating damage anywhere in the blade. Tribological damage includingplastic deformation, fretting, adhesive wear, oxidation, and phasetransformations, such as melting can occur at rubbing crack faces. C. J.Pye et al. (Ref 1) where cyclic gravitational loads pass throughstructural anomalies. Heat is generated by three mechanisms (J. Renshaw,Ref 2) internal frictional rubbing of contacting surfaces (asperities)on crack walls, deformation of the plastic zone surround the crack, andviscoelastic losses.

While the local blade surface temperature anomaly is small and the areamay be small, the present invention provides excellent signal to noiseration and quantitative data to evaluate the condition of the blades.Telephoto optics are necessary to resolve the small angle α subtended byfor example a 6 inch long crack at 300 ft. height, where α=arc sin1/600=0.0955° or only 343 arc seconds. Further, if the defect is locatednear the blade tip, the thermal indication will be moving at a high rateof speed. For example, on a typical 50M blade operating at 20 rpm, thethermal indication will be moving at 176.8 ft./second.

Depending on the characteristics of the thermal camera, which commonlyuses a relatively slow micro bolometer thermal sensor, the rotation ofthe blade through the field of view typical thermal camera makes itdifficult for an operator to see much less analyze the thermalindications, even if the camera has the thermal resolution performanceto detect the emissions at all. Positioning the thermal camera on theright side of the tower (on the ground or at sea level for an off shoreturbine with ones back to the wind) at a location approximately mid-spanunder the turbine rotor disk (the plane containing the blade tips)allows the camera to image the leading edges of the turbine blades asthey move directly towards the camera with the angle of the bladechanging relatively slowly. Good results for defects on the bladeleading edges can be obtained. The trailing edges may be inspected inthe same manner from the left side of the tower (with ones back to thewind) where the blades are moving directly away from the camera. Imaginga defect that is located elsewhere on the blades is more challengingsince the blades are viewed up or down wind of the turbine disk and theimage moves rapidly across the field of view of the camera.

The thermal images of the rotating blades from positions upwind ordownwind of the plane of the turbine disk can be captured with thethermal camera using the camera memory, if present, or a computer withthe appropriate drivers for capturing digital images using a GigE orcamera link interface or an analog to digital video frame grabber or anyother appropriate camera/computer interface well know in the art. Asoftware algorithm to display each frame by frame in a series ofsequenced images allows defects to be identified as the blade rotatesinto view of the camera. Part of this invention also includes apeak-store image function that records the stored maximum value for eachpixel in each frame. As a hot spot on the blade, caused by stressinduced internal friction at the site of a propagating defect,plasticity or thermoelastic emissions, rotates into view, each pixel islocked to its maximum value. With a series of sequential frames combinedinto one peak stored image the motion trajectories of defect indicationsare recorded through each blade pass. Because wind turbine blades arepainted to reflect heat from the sun, the heat from sources other thanactive defects in the blade may appear in the thermal images of bladesas reflections. On example might be the heat from a car situated nearthe wind tower. Heat signal from a defect will form a sine wavetrajectory track around the blade path, while reflection of heat sourceson the ground or on adjacent towers appear “painted” on the image of theblade through a small number of frames or a small angle of bladerotation.

In multiple embodiments of this invention a blade image de-rotationstabilization device can be added to the front aperture of the thermalcamera to derotate the motion of the blade as it passes through thecamera field of view.

A two axis mirror motion using two actuators would allow more preciseimage motion compensation as the blade is viewed from a distance of from100 feet to 1000 feet with frequently a considerable off-axis view. Assuch, the blade motion and the motion of any thermal emitters on theblade surface appears to move in a combined vertical and horizontalmotion approximating a sine wave. Blades are best inspected when withinapproximately 45 degrees of the 90 and 270 degree horizontal position sothat the images across the span of the blade have approximately the samescale.

The output signal from a function generator producing a ramp functioncan be used to drive a voice-coil linear actuator at an amplitude andrepetition rate required stop the appearance of motion for several videoframes. As the blade starts to pass through the field of view, themirror pivot motion starts and tracks the blade image across the fieldof view. The frequency and amplitude can be set with each cyclecorresponding to the turbine rotational period τ seconds to track oneblade or τ/3 to track all three blades sequentially. Other embodimentsinclude the use of the graphical target generated by software that isplace on the video image from the thermal camera that triggers thede-rotator to initiate a mirror motion cycle. As the blade enters thethermal camera field of view and crosses the graphical target on thedisplay, the computer starts to move the mirror to approximately trackthe blade as long as possible allowing the thermal camera time tocapture multiple high quality images. The de-rotator motion may also betriggered by a photo detector established to detect light from a laserbeam illuminating the blade from the ground. The unexpanded low powerlaser and detection electronics would provide an electronic triggersignal when the blade was in the correct position to start tracking.

In another embodiment, a light weight thermal camera can be mounted to ahinged plate and operated with any of the methods for image de-rotationof the rotation blade as describe herein.

Referring now to the drawings, wherein like reference numerals designatecorresponding structure throughout the views, and referring inparticular to FIG. 1, is a schematic diagram of a HAWT that is typicalof both land based and off-shore turbine generators. The view 1 frombehind the turbine facing the wind includes tower 6 extending up fromthe ground or ocean surface 28 to support the nacelle 8 which containsthe generator and gear reducers, unless it is a direct drive generator.There are typically three blades, 11, on a utility scale wind turbinehaving root ends 10 and blade tips 12. As seen from the side view 3, theblade root ends attach to the rotatable hub 18. Blade side 16 facing thewind 4 is often referred to as the high pressure side. The blade side14, facing away from the wind is referred to as the low pressure orsuction side. As the blade speed 17 increases, the blade pitch isadjusted to the optimal angle of attack to the wind 4 to create themaximum lift and torque required to drive the electricity generator.

FIG. 2 shows the construction cross section of a typical HAWT blade.Wind turbine blades are generally manufactured with adhesively bondedcomposite shells forming the high pressure side 16 and the low pressureside 14. The trailing edge 21 is formed by the adhesively bonded shells14 and 16 as is the leading edge 20, with adhesive bonding in some casesbetween two flanges 22 formed by the inner and outer fiberglass skinsthat make up sandwich panels 18. Two spar caps 26, which may be madefrom fiberglass or carbon fiber laminate or other composite material,are bonded to the edges of the sandwich panels 18. The blade spar web30, which can be a solid fiberglass laminate or a sandwich constructionwith fiberglass or carbon fiber face sheets and a core material madewith foam, balsa wood or other suitable material with high compressivestrength. The spar web 30 is bonded with adhesive 28 to the spar caps 26to form an I beam. Sometimes a second or even third spare web is presentforming a box beam. Defects such as adhesive disbonds or unbonds presentat the spar cap 26 to spar web 30 adhesive bond 28 may lead tocatastrophic failure of the blade in service. Fiber waves in the solidspar cap 26 laminate can also lead to cracking and ultimately to bladefailure. Further, trailing edge 21 splits or cracks in the high pressure16 and low 14 pressure side shell adhesive bond 24 may be signs orexcessive blade flex during operation. The trailing edge 21 adhesivebond 24, in the area of greatest blade chord width towards the root end10 supports blade twist loads. Cracks and breaks in the adhesive bond 24at these locations can also lead to blade failure unless detected intime and the turbine shut down and promptly repaired.

FIG. 3 is a schematic diagram showing the side view of thermal camera222 at locations down wind of tower 6 for inspection of the low pressureside of the blade 11. Thermal emissions 204 from defects in the blade 11propagating due to cyclic stress load do to gravity and windfluctuations are detected by thermal camera 222. The side view alsoshows the plane 20 as a line extending to the ground or sea level 28,containing the blade tips 12. This approximate location is a goodviewing position for blade 11 leading edge 20, inspections.

FIG. 4 shows thermal camera 222, positioned at locations both on the upwind-high pressure side of the turbine blades, at two locations oneither side of the tower in the plane of the blade disk and at alocation on the down wind, low pressure or suction side of the blades toprovide advantageous viewing of the entire blade surfaces duringrotation. For the best view angle of the low pressure side 14 of theblades 11 is achieved with the thermal camera 222 positioned atapproximately location 212, however the best results are obtained usingan image stabilizer as described here in or an image peak storage unitor software due to the relatively high speed the image of the bladepasses through the camera field of view. The best view of the bladeleading edges 20 are at position 214, in the plane 20 of the blade tips,since the blade 11 is rotating down directly at the camera and theangles of points on the leading edge 20 are changing relatively slowly.Moving down wind from position 214 gives good views of the forward lowpressure side surfaces 14, of the blade 11 with relatively low rate ofangle changes over three to four video frames. Position 216 is excellentfor the trailing edge 21 of the blades and position 220 offers a fullview of the high pressure side 16 of the blade 11, which is best viewedwith the image stabilizer or peak storage embodiment described herein.Position 210 offers relatively poor views the low pressure side exceptfor the trailing edge 21 and the aft low pressure sandwich structure 18,but with fast moving angle changes and with the blade twist. Windturbine farms are often located on hills or mountain ridges with littleroom to move away from the tower 6. Local conditions and land shapesoften dictate where the best viewing angles can be obtained.

FIG. 5 is a schematic diagram showing a thermal camera 222, positionedto receive low level thermal radiation 228 from the blade 11 due tothermoelastic emission from the stresses acting on the blade materialdue to gravitational forces from the blade rotation motion 17.

Emission 226 from the mechanically stressed defect 224 appears warmer inthe image produced by thermal camera 222 due to internal friction andplasticity around defect 224.

The camera 222 is positioned under the blade at position 214 to receivethermal radiation 228 from the leading edge and the forward part of thelow pressure side 14. This position reduces the angular changes due toblade rotation in the image during the frame acquisition. The streamedvideo images from the thermal camera 222 are recorded by ImageProcessing computer 230, or in a memory device in the thermal camera 222as video files, and processed presented using peak store or other imageprocessing techniques and presented on monitor 232. Various means ofprocessing the images including video image peak store, frame by frameanalysis, histogram normalization, unsharp filters and so on to obtaingood image quality and quantitative measurements of size and locationcomparing features of known size at the range to the target.

FIG. 6 shows how a thermal camera located at position 212 can be used totest the entire blade from one location by pointing to a blade sectioninboard adjacent to the generator nacelle 8 and acquiring the thermalimage sequences. The moving the camera in an arc 230 outboard andoverlapping the next blade section and so on until the tip is reached.The use of programmed servo or other motor drives to move the camerawould allow for a fast automated test.

FIG. 7 shows a representation of the video peak store function. Thethermal emitting defects 236, 238 and 240 on blade 11 as shown, rotatecounter-clockwise in this view of the low pressure side 14 of blade 11,as seen from down wind of tower 6. The axis of rotation 7 is shown inthe approximate middle of the nacelle 8. The larger thermal emittingdefect, 236, happens to be the closest defect to the axis 7 andgenerates the strongest signal 248. In a peak stored image, each pixelis locked to the highest gray level value while image data is beingacquired by the post-test analysis software or in the field during thetest. The result is sources of thermal emissions can be tracked throughthe field of view as show in defect traces 248, 246 and 244. and asshown in FIG. 13. A thermal emitting defect has a trajectory followingthe blade in rotation as viewed greatly off-axis. A line scan, 252through the peak stored image 242, maps the gray level value for eachpixel in a line from one side of the image to the other and allows agraph 254 to be plotted showing the gray level signal intensity vs.position on the blade. The distance scale is calibrated in software fromthe size of know features on the nacelle 8 or tower 6. These values canbe corrected mathematically for the difference in velocity for differentspan wise locations on the blade using a look up table (LUT) or othertechniques for correcting image data based on geometric parameters wellknow in scientific programming and photogrametry.

FIG. 8 shows one embodiment of the thermal image de-rotator consistingof a mirror 241, attached to a hinge 250 at one end and the opposite endis connected to a linear actuator or voice coil linear actuator 238through a ball joint or universal joint 240, whose cyclic motion in thedirection of the blade motion can be adjusted to track the approximatemotion of each blade 11 as it rotates through the field of view. Thederotation will essentially stop the rotation motion for several frames.The blade appears to hang in space giving the thermal camera time togenerate higher resolution images of the moving blade. The adjustmentsconsist of amplitude of the ramp voltage 260, which will compensate forthe speed of blade rotation, the duration of the ramp function 262, andthe time 264, between each start of the ramp function. The output offunction generator 244 is amplified in amplifier 242 which then drivesthe linear motor actuator 238. The motion of mirror 241 can de-rotateshort sequences of images produced by thermal camera 222. By selectingthe repetition rate, 262, the user can select to image all the blades bystarting the tracking motion every τ/3 seconds, where τ=the period ofrotation of the turbine in seconds or by selecting a repetition rate ofτ, the images of the same blade 14, will be presented and de-rotated. Aspring can be used to provide a restoring force to bring the mirror backto its start position, ready for the next blade pass. Only one or morevideo frames need to be tracked to substantially improve blurring of thethermal camera image due to blade rotation. The frequency of motion ofthe de-rotating mirror 241, can be calculated in the following example.Assume a turbine operating @ 15 rpm, a thermal camera 222 with a 30frames per second frame rate, and it is desired to de-rotate 4 videoframes. The period, , of the turbine is the time required to make 1revolution, in this case 60 seconds/15 rpm=4 seconds. In this time, thethermal camera 222 (operating at 30 frames/second) captures (4seconds×30 frames/second)=120 frames. The 4 frames we wish to de-rotateare captured over a time of (4 frames/30 frames/sec.)=0.133 seconds. Tode-rotate these four frames then, we start moving the mirror to trackthe blade starting at 0 seconds, when the blade 14 enters the field ofview of the thermal camera. The mirror 241, angle is changed (increasedor decreased depending on what direction the blade is moving through thefield of view) continuously over 0.133 seconds, then returned to thestart position. To track only one blade, the tracking motion is startedevery 4 seconds (τ). To track all three blades sequentially, thetracking motion is started every τ/3 seconds, 4/3=1.33 seconds. Aftereach tracking motion, the mirror is returned to the start position toawait the next tracking cycle.

FIG. 9 shows a second embodiment where the thermal camera 222 is movedin rotation to follow the blade 11 motion around axis 7. The thermalcamera 222 is mounted on a frame or plate 251 and connected to hinge 252which is also attached to frame member 250 which supports actuator 238.The opposite end of plate 251 is connected to the electric actuatorthrough a flexible joint, such as a hinge or universal joint. Themovement of the actuator causes the thermal camera 222 to move up anddown which when aligned with a portion of the blade here the motion issubstantially the same, will tend to stabilize the blade in the field ofview of the thermal camera 222. Additional actuators can operatesimultaneously to move the thermal camera in multiple directions howeverwith additional complexity. In practice, one actuator is sufficient if aclear view of the blade 11 when it reaches the horizontal positionduring rotation can be attained in the field. Another option is torotate the entire actuator 238, support plates 250 and 251 as well asthe thermal camera 222 to align the motion of the actuator with themotion of the blade during data acquisition. Reducing the driven massreduced vibration and power requirements for the electronics andelectric actuator, which may be battery operated or power from thevehicle used by the operator. The motion of the camera should be alignapproximately with the direction as the motion of the turbine blade 11in the camera field of view during it's rotation on axis 7.

As the high pressure side of the blade, which viewed from the up windside of the tower 6 at position 220 is turning clockwise, rises into thefield of view of camera 222, the leading edge of the blade eventuallycrosses threshold marker 242. This motion can be identified as a changein the pixel intensity value from that of the open sky as seen bythermal camera 222, and the software electronically triggers the startof the motion of linear actuator 238 by computer 230 the waveformgenerator 244 and amplifier 242. Again, an optimal waveform is a rampfunction which will move the camera 222 at a constant angular velocityto receive in lens 228 the thermal emissions 226 from the defect in theblade 224 as it moves vertically across the field of view at a constantvelocity. This embodiment can also use a moving move a mirror. Thetripod mount can also be fitted with a mechanical azimuth and elevationfor example using rod 258 connected with a flexible joint 254 to thebase frame for the camera 250 and locked with clamp 260 with the linearmotor providing fine movement of the thermal camera or a mirror.Although the embodiment shown uses a tripod 204 to support the thermalcamera and image de-rotation mechanism, any sturdy support can be usedincluding a truck, van, car or a wheeled cart to easily move theequipment around the site of the wind turbine. FIG. 10 shows a graph ofvoltage vs time for the signal used to drive the image de-rotationactuator. For the manually operated de-rotation embodiment, the functiongenerator operates continuously. The operator adjusts the repetitionrate to match the turbine rotation period τ, 268. The ramp functionshown in FIG. 10 has a 0.133 second duration to match the time for fourvideo frames to be displayed. The electronic or software system can bedesigned to allow the operator to input these values directly and havethe de-rotation device, using either the moving mirror 241 or the movingthermal camera configurations.

FIG. 11 is a schematic drawing showing the addition of a gimbal mount tostabilize the thermal camera 222, and it's various motion mechanisms andembodiments described here in, during use on water aboard a ship orvessel to inspect blades 11 mounted on off shore wind power generators.In one embodiment, counter weight 280 is supported by gimbal mountcomprised of inner frame 270 and outer frame 272 connected by bearingpins 274 and 276, as well as support housing or frame members 278 whichis supported on the deck 29 of the ship or vessel. This allows camera222 to remain aligned with blade 11 during inspections. This gimbalassembly may also be actively powered and use servo actuators andaccelerometers, well know in the art, to provide a stabile platform withrespect to the vessel rolling and pitching movement for successfuloperation of the blade inspection system described herein. In addition,the shipboard blade test system may use an inertia platform to keep thecamera aimed at the target blade during tests to compensate for shipmotion. FIG. 12 is a schematic diagram of the wind turbine bladeinspection system described herein with the addition of a videorecorder, camcorder, photographic camera or video camera 242 and a lightsource 244, configured to record the serial numbers 282 written on theroot end 10 of each blade as it is tested. Frequently the optimalposition for the thermal camera during the test is not the same positionneed to place the serial number camera to image the serial numbers. Anyone of a number techniques may be used to synchronize the thermal imageswith the serial number images. First, a GPS clock and provide timingsignals on the sound track of a video camcorder as well as timingsignals to the thermal camera 222 or the computer 230, if connected atthe time of data acquisition. Second, if the image de-rotator is beingused an audible or electrical or visual signal can be sent by electricalline 243 to the serial number camera when the actuator begins to trackthe blade 11. A delay may be needed to time the serial number capturewith the position of the blade to allow the blade to move from the testangle to the best angle to capture the correct serial number.

FIG. 13 shows two test results on operating 1.5 MW utility scale windpower generators made with embodiments of this invention. The top imageshows seven video frames of one blade from the high pressure side as itrotates through the thermal camera field of view. The images wererecorded as an .MOV file and played back through software that generatesa peak store image. The trajectory 246 of the thermal emissions from anactive propagating defect is seen as the blade rotates from top tobottom clockwise. The dark lines in the image of the high pressure side14 of this blade are thermal camera 222 artifacts due to the fast blade11 motion. The bottom image shows a test image of a leading edge ofblade 11 that includes thermal response from hub 18 nacelle 8 and tower6. The test was made from position 214 in FIG. 4. Two indications ofdefects 236 are seen. Scaling from known features in the image, thedefects are located at 7.2 and 9 meters from the root end 10.

It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meaning of the terms inwhich the appended claims are expressed.

What is claimed is:
 1. A method of remotely inspecting a wind turbineblade in situ, comprising: imaging a portion of a wind turbine bladewith a thermal imaging camera when the blade has substantially reachedthermal equilibrium with ambient air after sunset in order to producethermal imaging data; analyzing the thermal imaging data to identifyatypical thermal patterns that could be indicative of frictional heatingwithin or around defects within the wind turbine blade.
 2. A method ofremotely inspecting a wind turbine blade according to claim 1, where thestep of imaging the portion of the wind turbine blade is performed whilethe turbine is rotating and undergoing cyclic load is due to gravity andvariations in wind loads.
 3. A method of remotely inspecting a windturbine blade or blades according to claim 2, the step of imaging theportion of the wind turbine blade includes acquiring a continuoussequence of images of more than one blade as it passes through the fieldof view of the thermal imaging camera.
 4. A method of remotelyinspecting a wind turbine blade or blades according to claim 3, furthercomprising a step of recording the images and creating a composite imageof a selection of sequential blade images with a video peak-storeprogram to track the motion of the source of thermal emissions todistinguish between an actual thermal emitting area on the blade fromunrelated thermal effects.
 5. A method of remotely inspecting a windturbine blade or blades according to claim 4, wherein a single image ora continuous series of images of each blade is recorded as it passesthrough the field of view of the thermal camera, the thermal radiationbeing directed to the camera aperture by an image derotation deviceconsisting of a moving mirror driven by an actuator to compensate forblade motion and resulting image degrading effects.
 6. A method ofremotely inspecting a wind turbine blade or blades according to claim 5,wherein the mirror is driven by an electric actuator using an functiongenerator with manual controls for frequency, wave form and amplitude.7. A method of remotely inspecting a wind turbine blade or bladesaccording to claim 4, wherein a single image or a continuous series ofimages of each blade is recorded as it passes through the field of viewof the thermal camera, the thermal radiation being directed to thecamera aperture by an image derotation device consisting of a movingmirror driven in multiple axes by multiple actuators to move a mirror tocompensate for blade motion wherein the electric actuator is driven withtwo separate waveforms from two function generators with manual controlsfor frequency, wave form and amplitude.
 8. A method of remotelyinspecting a wind turbine blade or blades according to claim 7, whereinthe waveform is a ramp or saw-tooth function.
 9. A method of remotelyinspecting a wind turbine blade or blades according to claim 4, whereina single image or a continuous series of images of each blade isrecorded as it passes through the field of view of the thermal camera,the thermal radiation being directed to the camera aperture by an imagederotation device consisting of a moving mirror driven in multiple axesand multiple actuators using an microprocessor or computer generatedwaveform.
 10. A method of remotely inspecting a wind turbine blade orblades according to claim 4, wherein a single image or a continuousseries of images of each blade is recorded as it passes through thefield of view of the thermal camera, the thermal radiation beingdirected to the camera aperture by an image derotation device consistingof a moving mirror driven by multi-axis actuator to move a mirror tocompensate for blade motion wherein the mirror is moved by an electricactuator using an microprocessor or computer generated waveform with anexternal trigger device to detect the blade position and initiate bladetracking thereby reducing degradation of the thermal image of the bladedue to rotation.
 11. A method of remotely inspecting a wind turbineblade or blades according to claim 4, wherein a single image or acontinuous series of images of each blade is recorded as it passesthrough the field of view of the thermal camera, the thermal radiationbeing directed to the camera aperture by an image derotation deviceconsisting of a moving mirror driven by single actuator or multi-axisactuators to compensate for blade motion wherein the actuator(s) is(are)driven with electronically generated waveform with an electronic triggersignal from the thermal camera indicating the image of the rotatingblade is entering the field of view of the thermal camera and initiatemirror actuation and blade tracking thereby reducing degradation of thethermal image of the blade due to said rotation.
 12. A method ofremotely inspecting a wind turbine blade or blades according to claim 4,wherein a single image or a continuous series of images of each blade isrecorded as it passes through the field of view of the thermal camera,whereby the thermal camera is mounted on a hinged supporting member witha vertical tilt actuator and drive electronics consisting of a waveformgenerators and amplifier to approximately track the blade image during aportion of the rotation thereby reducing the degrading effects of blademotion on the thermal camera image.
 13. A method of remotely inspectinga wind turbine blade or blades according to claim 4, wherein a singleimage or a continuous series of images of each blade is recorded as itpasses through the field of view of the thermal camera, whereby thethermal camera is mounted on a hinged supporting member with a verticaltilt actuator with drive electronics consisting of a waveform generatorsand amplifier to track the blade image during a portion of the angularrotation with an electronic trigger signal from the thermal cameraindicating the image of the rotating blade is entering the field of viewof the thermal camera and to initiate a vertical camera actuation and toapproximately track the blade thereby reducing degradation of thethermal image of the blade due to said rotation.
 14. A method ofremotely inspecting a wind turbine blade or blades according to claim 4,wherein a single image or a continuous series of images of each blade isrecorded as it passes through the field of view of the thermal camera,whereby the thermal camera is mounted on a hinged supporting member witha vertical tilt actuator and a horizontal pan actuator with driveelectronics consisting of two waveform generators and amplifiers totrack a portion of the blade image during rotation thereby reducing thethermal image degrading effects of blade motion.
 15. A method ofremotely inspecting a wind turbine blade or blades according to claim 4,wherein a single image or a continuous series of images of each blade isrecorded as it passes through the field of view of the thermal camera,whereby the thermal camera is mounted on a hinged supporting member witha vertical tilt actuator and a horizontal pan actuator with driveelectronics consisting of two waveform generators and amplifiers with anelectronic trigger signal from the thermal camera indicating the imageof the rotating blade is entering the field of view of the thermalcamera and initiate camera motion actuation and approximate bladetracking thereby reducing degradation of the thermal image of the bladedue to said rotation.
 16. A method of remotely inspecting a wind turbineblade according to claim 4, wherein a video image of the blade serialnumbers on the root end of the blade is recorded continuously with avideo camera and an appropriate light illumination source andsynchronized with video frames from a thermal camera imaging the bladefor anomalies by using a sound signal that is recorded by the videocamera and triggered manually by the operator when the defect appears onthe video screen of the IR camera or computer thereby identifying theserial number of the blade with detected defects.
 17. A method ofremotely inspecting a wind turbine blade according to claim 4 wherein avideo image of the an illuminated view of the blade serial numbers onthe root end of the blade are recorded and synchronized with videoframes from a thermal camera imaging the blade for anomalies, using GPStiming signals.